The present invention relates to knock detecting apparatus, and particularly to a knock detecting apparatus suitable for use with an ignition timing control apparatus for an internal combustion engine, which detects knocking from the vibrations emitted to the outside of the engine cylinder due to the pressure therein and the ignition timing of the engine is adjusted so as to obtain a desired knock intensity.
It is well known in the art that there is a close correlation between the ignition timing and the cylinder pressure. When the mixture is exploded in the absence of any knock harmonic components (usually those frequency components in the range between 5 and 10 kHz, and in the frequency band determined by the bore diameter of the engine cylinder and the velocity of sound in the combustion and which is produced as the result of intermittent and rapid combustion) will not be superpositioned on the cylinder pressure. On the contrary, when the engine starts knocking, such high frequency components start to be superposed on the cylinder pressure at around its peak value and this has the effect of causing emission of vibrations or sound to the outside of the cylinder. A close examination of the pressure in the cylinders or the vibrations or sound emitted to the outside of the cylinders will show that the beginning of knock (trace knock) occurs at the engine crank angle at which the cylinder pressure attains the maximum value. As knocking graduallyincreases (light knock and heavy knock), the resulting harmonic components on the cylinder pressure gradually appear at an earlier crank angle than that corresponding to the maximum cylinder pressure value (or on the ignition side). Thus, the engine efficiency will be increased greatly if the vibrations or sound emitted by knocking to the outside of the cylinders is detected with a high degree of accuracy and fed back to control the ignition timing. However, presently no detecting apparatus has been proposed which is capable of detecting the presence of knock or feedback factor with improved accuracy and which is also operable stably under severe environmental conditions required for vehicles.
Prior art knock detecting apparatus of the above type include those which employ piezoelectric type accelerometers for detecting knocking vibrations generated in the engine. This accelerometer is required to meet the characteristic requirements of the knocking detector for internal combustion engine, but is disadvantageous in the following points:
(1) Since the timing at which knocking can be detected in the internal combustion engine becomes earlier as the level of knocking becomes greater, the knocking level at which ignition timing is adjusted should be less than the trace knock level and the detection must be performed with high precision and stability. The piezoelectric element has essentially a high impedance and is liable to be affected by moisture and dust so that the very small signal associated with trace knocking is difficult to produce stably (due to increased drift and fluctuations). The piezoelectric characteristic of the element occurs because of polarization and the operating conditions of the combustion engine greatly change cyclically from high temperature to low temperature, so that this polarization is gradually decreased thereby reducing the sensitivity of the piezoelectric element. The decrease of the sensitivity of the piezoelectric element results in the increase of the knocking control level so that the apparatus is responsive only to a strong knocking, and thus the engine is sometimes damaged by the knock. PA1 (2) The detector should be inexpensive too. However the piezoelectric element has a high impedance and its output must be amplified by an expensive amplifier having a high input impedance called a charge amplifier. In addition, this high-impedance amplifier is liable to be affected and misoperated by noise due to ignition signal of the internal combustion engine, and hence it is further necessary to use an expensive, complicated arrangement for the prevention of the influence. PA1 (3) The piezoelectric element has a weak impact resistance, is liable to be cracked or broken off, and requires a cooling means for use at a high temperature, and thus it is unsuitable for a detector on a vehicle from the viewpoint of durability and cost.